1. Technical Field
This invention relates to the control of mist formation above electrolyte solutions during processes, such as the electrowinning, electroplating, and electroforming of metals, in which a potentially-hazardous mist is formed.
2. Background and Related Art
Electrowinning is the process by which metals are recovered from aqueous electrolyte solutions resulting from the extraction of the metal ion from an acidic or basic leach solution. It is frequently employed in the recovery of metals, such as copper, from the respective metal-containing ores, wherein the leaching, solvent extraction, stripping and electrowinning of metals from their ores have many of the same common unit operations or steps. These steps frequently include: (1) the metal value(s) in the mined and crushed ores being converted to an acid-soluble form, possibly by an oxidizing roast or a reduction; (2) ores from step (1) being leached, such as with an aqueous solution of a strong acid, usually sulfuric acid, to form an aqueous acid leach solution of pH 0.9 to 2.0, containing the desired metal ion and relatively small quantities of other metal ions (impurities that must be removed prior to final recovery of the desired metal(s)); (3) the resulting metal(s)-pregnant aqueous acid leach solution being mixed in tanks (possibly with one or more repetitions in order to improve metal recovery and/or to separate desired metal value(s)) with an extraction reagent, such as an oxime or mixture of oximes, that is selective for the desired metal(s), and dissolved in a water-insoluble, water-immiscible organic solvent, to form a metal-extractant complex/chelate that is separated from the metal(s)-depleted aqueous phase in, e.g., a settling tank; (4) the metal-loaded organic phase is then mixed (again, possibly, with one or more repetitions) with a highly-acidic strip solution (e.g., concentrated sulfuric acid), which breaks apart the complex, dissolving the metal ions into another aqueous solution that, following another phase separation from the now-metal-depleted organic phase, is customarily forwarded to an electrowinning “tankhouse”; and (5) in the tankhouse, the metal values are deposited on the cathodes by electrodeposition and then recovered from those plates. Other processes may be employed with other metals, such as nickel, zinc, and the like, in order to produce an electrolyte from which their respective metal values may be electrowon.
It is during the electrowinning (electrodeposition) stage that an acidic mist is often generated above the electrolyte (strip aqueous phase). This mist is a result of small bubbles of oxygen being generated at the anode, while the metal is plating out at the cathode, and when these bubbles rise to the top of the electrolyte solution and break, small particles of acidic electrolyte are shot into the air, resulting in an acidic mist.
Electroplating is the process of applying a metallic coating to an article by passing an electric current through an electrolyte in contact with such article. The ASTM adds some quality restriction by defining electroplating as electrodeposition of an adherent metallic coating on an electrode such that a surface having properties or dimensions different from those of the basic metal is formed.
In the electroplating process, the metals or metalloids (nonmetals that are semiconductors, e.g., arsenic, germanium, and the like, which may be electroplated in the same manner as metals), being used may be present in the aqueous compositions in metallic form and/or in an anionic form, may be one or more of zinc, nickel, copper, chromium, manganese, iron, cobalt, gallium, germanium, arsenic, selenium, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, lead, bismuth, mercury, antimony, gold, iridium, and/or platinum. In addition, many alloys, such as brass, bronze, many gold alloys, lead-tin, nickel-iron, nickel-cobalt, nickel-phosphorous, tin-nickel, tin-zinc, zinc-nickel, zinc-cobalt, and zinc-iron, even lead-indium, nickel-manganese, nickel-tungsten, palladium alloys, silver alloys, and zinc-manganese, are also commercially electroplated.
Another type of electrodeposition in commercial use is a composite form, in which insoluble materials are codeposited along with the electrodeposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity. Silicon carbide and other hard particles, including diamond, are co-deposited with nickel in order to improve wear properties or to make cutting and grinding tools.
The essential components of an electroplating process are an electrode to be plated (the cathode); a second electrode to complete the circuit (the anode); an electrolyte containing the metal ions to be deposited; and a d-c power source. The electrodes are immersed in the electrolyte, such that the anode is connected to the positive leg of the power supply and the cathode to the negative. As the current is increased from zero, a minimum point is reached where metal plating begins to take place on the cathode.
Plating tanks are formed from materials which are either totally inert to the plating solution or are lined with inert materials in order to protect the tank. For alkaline plating solutions, mild steel materials are used. For acid plating solutions, other materials are used, depending on the chemical composition of the plating bath, such as titanium and various stainless steel alloys, polytetrafluoroethylene, KARBATE® impervious graphite, HASTALLOY® nickel alloys, zirconium alloys, and the like.
The plating tanks are fitted for d-c power, usually with round copper busbars, with filters to remove fine particulate matter. Heating or cooling units may be present, employing heating coils or cooling water coils, and two types of anodes may be used, i.e., soluble or insoluble (when insoluble anodes are used, the pH of the plating solution decreases along with the metal ion concentration, and in some plating baths, a portion of the anodes are replaced with insoluble anodes in order to prevent metal ion buildup or to reduce metal ion concentration). See, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, 4TH Edition, under the heading, “Electroplating”.
Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating, generally requiring that substrates being electroplated be prepared prior to electroplating. In the effort to get the substrate into this condition, several separate steps may be required, such as soak cleaning, followed by electrocleaning, followed by rinsing.
Formulations of plating baths may be flexible in some systems and very sensitive to variations in others, many of the more recent changes resulting from waste treatment and safety requirements. Besides the ability to deposit a coating having acceptable appearance and physical properties, the desired properties of the plating bath include: high metal solubility, good electrical conductivity, good current efficiencies for anode and cathode, noncorrosivity to substrates, nonfuming, stable, low hazard, low anode dissolution during down-time, good throwing power, good covering power, wide current density plating range, ease of waste treatment, and economical to use. Few formulas have all these attributes, with only a few plating solutions being used commercially without special additives, although chemical costs often constitute a relatively low percentage of the total cost of electroplating. Such additives are used in these solutions to brighten, reduce pitting, and/or otherwise modify the character of the deposit or performance of the solution to meet some of the criteria above, with the suppliers of the proprietary additives normally specifying the preferred formulations to be used.
Purification, often needed once a plating bath is prepared, is used periodically to maintain the plating solutions. Alkaline zinc plating solutions are sensitive to a few mg/L of heavy-metal contamination, which may be precipitated using sodium sulfide and subsequently filtered out. Nickel plating solutions may contain excess iron, which may be removed by peroxide oxidation, precipitated at a pH of about 5, and filtered out. The more complex, less water-soluble organic contaminants, along with some trace metals, are removed with activated carbon treatments in separate treatment tanks. A common purification treatment used both on new and used plating solution is dummying, in which heavy-metal impurities are removed by electrolyzing, usually at low current densities, using large disposable steel cathodes, good agitation and lower pH speeding the process.
Relatively simple analyses and testing, requiring little equipment, are required whenever a new plating solution is made up, and thereafter at periodic intervals. Trace metal contaminants may be analyzed by using spot tests, colorimetrically, and with atomic absorption spectrophotometry. Additives, chemical balance, impurity effects, and many other variables are tested with small plating cells, such as Hull Cells.
The precise makeup of plating bath compositions depends on the metal being plated. For example, cyanide copper plating baths typically contain copper metal, copper cyanide, potassium cyanide, potassium hydroxide, Rochelle salts, and sodium carbonate, while acid copper plating baths typically contain copper metal, copper sulfate, sulfuric acid, and various additives, and watts nickel plating baths typically contain nickel metal, nickel sulfate, nickel chloride, boric acid, and various additives, while sulfamate nickel plating baths contain nickel sulfamate instead of nickel sulfate.
Electroforming is the production or reproduction of articles by electrodeposition upon a permanent or expendable mandrel or mold that is subsequently separated from the electrodeposit, with the electrodeposit becoming the manufactured article. Of all the metals, copper and nickel are most widely used in electroforming.
A problem common to all of the above electrolysis procedures is the presence of mist-acidic or alkaline-generated above the electrolyte solutions. Such mist is a severe health hazard and causes corrosion to the plant facilities and operating equipment. In order to reduce the quantity of mist, anti-misting agents (also referred to as demisting and mist-suppressing agents) are commonly added to the electrolyte solutions. However, the currently-available anti-misting agents are not completely satisfactory, due to limited demisting ability, high loss rate of such anti-misting agents, interference with the electrolysis process, and/or ecological incompatibilities. In the case of electrowinning, acid mist is particularly noteworthy and troublesome, and the use of mist-suppressing agents can interfere with the related extraction process (e.g., due to partial solubility in the organic extraction solution, promotion of emulsion formation, promotion of slow phase separation, and interference with extraction or stripping kinetics). In addition, foaming can also be a significant problem.
Tjernlund, D. M., et al., in “A Study of the Fire and Explosion Hazards Associated with the Electrowinning of Copper in Arizona Surface Mine Plants”, Section 8, U.S. Department of Labor, Mine Safety and Health Administration, Investigative Report, March 1999, report on the formation of acid mist in electrowinning as follows: “Oxygen bubbles created at the anode rise to the surface of the electrolyte. At the surface, these bubbles expand above the liquid and then break, releasing entrapped oxygen into the atmosphere. The liquid in the bubble wall just before it breaks is made up of the acid electrolyte solution. As the liquid wall ruptures, it disintegrates into extremely small droplets that readily become airborne. The macroscopic effect of this process is to create an acrid acid mist above the cells. This mist readily migrates throughout the workplace and represents a potential health hazard to workers in the tankhouse. It also creates a corrosive atmosphere that can be detrimental to equipment and the tankhouse structure itself.” Under Section 8c, “Strategy 4: surfactants”, the authors go on: “In most tankhouses, a water-soluble surface tension reducer is used to discourage misting . . . . By lowering the electrolyte surface tension, the gas bubble wall becomes thinner when it reaches and protrudes above the electrolyte surface. This causes the bubble to break sooner with less generation of mist droplets”.
The authors note further that 3M's FC-100 and FC-1100 FLOURAD™ were commonly-used anti-misting agents. They reported that FC-100 trapped the rising gas in soapsuds-like bubbles above the surface, even at very low concentrations (a few hundred ppm). They note further, however, that the resulting foamy layer also created significant potential fire/explosion problems. They included that the FC-1100 surfactant had significantly less tendency to form suds than FC-100, but that, at higher concentrations, it, too, generated an undesirable foam suds layer.
3M states that FC-1100 FLOURAD™ contains 45-55% fluorochemical solids and 45-55% water and provides sulfuric acid mist suppression in the copper electrowinning tankhouse without the formation of a stable foam blanket at the surface of the electrowinning cell. The exact nature of the fluorochemical solid is not disclosed.
C. Y. Cheng et al, “Evaluation of Saponins as Acid Mist Suppressants in Zinc Electrowinning”, Hydrometallurgy, vol. 73 (2004), pp. 133-145, report on two saponins-rich products (Mistop® Quillaja saponaria extract and QLZinc® saponin) and include the the use of Mistop® saponins in a commercial copper electrowinning operation. The authors report that saponins (high-molecular weight glycosides of steroids, steroid alkaloids or triterpenes found in plants, consisting of a sugar moiety linked to a triterpene or steroidal aglycone, often referred to as nonrefined Quillaja extracts) are natural surface-active compounds that give stable foams in aqueous solutions.
U.S. Pat. No. 6,833,479 B2 (Witschger et al), incorporated herein by reference in its entirety, discloses anti-misting agents that are alkoxy-capped amine and trialkylol compounds having the structure: R((AO)nHmHp (formula (a)), wherein each AO group is, independently, an alkyleneoxy group selected from ethyleneoxy, 1,2-propyleneoxy, 1,2-butyleneoxy, and styryleneoxy groups; n is an integer of from 2 to 100; m is an integer of from 1 to the total number of —OH plus —NH hydrogens in the R group prior to alkoxylation; the sum of m plus p equals the number of —OH plus —NH hydrogens in the R group prior to alkoxylation; and the R group is a group selected from compounds of nine formulas, of which two of them are: N(CH2CH2O)3 (b) and CH3CH2C(CH2O)3 (c). Otherwise, the R group of four other formulas represents amine derivatives, as exemplified by formula (b) above; the R group of another formula represents alkoxylated trimethylol-ethane or -propane compounds, as exemplified by formula (c) above; the R group of still another formula represents alkoxylated pentaerythritols; and the final R group alternative represents alkoxylated phenylenediamine.
In testing, however, the monoethanolamine derivative of the particularly-preferred triethanolamines of formula (b) of this reference, when they contain six propylene oxide groups and eleven ethylene oxide groups showed unacceptable interference with the copper electrowinning process, in that, with its use, nodules formed on the necessarily-smooth surface of the cathode. Nodule formation is particularly undesirable, as nodules can grow to the extent that they physically touch the anode, resulting in a direct electrical short in the electrowinning cell, and/or they tend to promote the entrapment of impurities in the copper deposited on the cathode, resulting in poorer quality of the recovered copper.
Additionally, test work carried out with another compound of U.S. Pat. No. 6,833,479, showed that approximately half of that anti-misting agent was extracted from the aqueous phase into the organic phase during stripping, potentially resulting in a buildup of the surfactant in the organic phase and eventual phase separation problems. The presence of these particular surfactants also adversely affected extraction kinetics. In view of these negative impacts, these types of anti-misting reagents are unacceptable for use in systems involving copper solvent extraction followed by electrowinning.
Extensive research has been devoted to reducing the mist during the electrowinning, electroplating, and electroforming processes, especially in electrowinning processes in which aqueous acidic electrolyte solutions of metal ions are typically used in the electrowinning step. By far the most common solution is to add an anti-misting agent to reduce the mist. However, the currently-available anti-misting agents are not completely satisfactory, there still being a need for improved anti-misting agents that: are ecologically compatible; are effective even at low concentrations; have a low loss rate; are compatible with the other plating bath chemicals and additives; and yet, do not interfere with the kinetics of metal stripping or phase separation in the metal recovery process if the anti-misting agent is present during these steps.
Thus, there still exists a need for improved anti-misting agents for electrolysis/electrodeposition, particularly with respect to the electrowinning of copper.